1. Field of the Invention
This invention relates to an image pickup apparatus such as a video camera or the like.
2. Description of the Related Art
In the field of image pickup apparatuses such as video cameras or the like, efforts have been exerted to automate operations and diversify functions in every respect, including an exposure setting action, a focus adjusting action, white balance control, etc. As a result, it has become possible to adequately pick up images irrespective of ambient shooting conditions.
With respect to a focus control device and a white balance correcting device which is arranged in a video camera or the like to control the gain of a color signal according to changes in color temperature of an object of shooting, a method has been contrived to detect the state of an iris mechanism which is used for controlling the quantity of light incident on an image sensor and to perform an optimum control by using the data of detection thus obtained.
The conventional image pickup apparatus has been arranged as shown in FIG. 1. Referring to FIG. 1, an image pickup optical system 1 is arranged to form an image of light coming from an object of shooting. An iris (hereinafter referred to as an iris) 2 is arranged to control the quantity of light coming from the object. A motor 3 is arranged to drive a focus lens included in the image pickup optical system 1. An iris encoder element 4 which is a position encoder and is formed by a Hall element, etc., is arranged to detect the state of the iris 2. A motor 5 is arranged to drive the iris 2. An image sensor 6 which is formed by a CCD, etc., is arranged to photoelectrically convert the light of an image picked up. A sample-and-hold (S/H) circuit 7 is arranged to sample the output (a picked-up image signal) of the image sensor 6. Reference numeral 8 denotes an automatic gain control (AGC) circuit 8. A driving circuit 9 is arranged to drive the iris driving motor 5. An iris control circuit 10 is arranged to control the iris 2 in such a way as to bring the average output level of the image sensor 6 within a predetermined level range. An AGC control circuit 11 is arranged to adjust the gain of the AGC circuit 8 to make the level of the picked-up image signal constant. A color separation circuit 12 is arranged to separate the picked-up image signal into R (red), G (green) and B (blue) color signals. Amplifiers 13a and 13b for white balance control are arranged to amplify the R and B color signals with the respective controllable variable gains. A process and matrix circuit 14 is arranged to form a luminance signal Y and color-difference signals R-Y and B-Y. An encoder circuit 15 is arranged to form a standard TV signal. A system control circuit 16 which is arranged to control the whole image pickup apparatus includes, as main components among others, a white balance control circuit 16a and a focus control circuit (AF device) 16b which is arranged to detect the level, etc., of a high-frequency component of the luminance signal Y and to control the focus lens in such a way as to bring the level of the high-frequency component to a maximum level. A driving circuit 17 is arranged to drive the focus lens driving motor 3.
With the apparatus arranged in the above-stated manner, the quantity of incident light of the object obtained through the image pickup optical system 1 is adjusted by the iris 2. The quantity-adjusted light is photoelectrically converted by the image sensor 6. A video signal outputted from the image sensor 6 is sampled by the sample-and-hold (S/H) circuit 7. The sampled video signal is amplified by the AGC circuit 8 to such a state that enables a next signal processing circuit to adequately carry out a signal processing action on the video signal. The video signal thus amplified is supplied to the color separation circuit 12, which separates the video signal into three primary color signals including R, G and B color signals. Of these color signals, the R and B color signals are supplied via the amplifiers 13a and 13b for white balance control (WB) to the process and matrix circuit 14. The G color signal is supplied directly to the process and matrix circuit 14. The process and matrix circuit 14 then forms the luminance signal Y and the color-difference signals R-Y and B-Y. The luminance signal Y and the color-difference signals R-Y and B-Y are supplied to the encoder circuit 15 to be converted into a video signal which is in the form of a TV signal.
Since the luminance signal Y and the color-difference signals R-Y and B-Y are to be used also for white balance control and focus control, these signals are supplied also to the system control circuit 16. The system control circuit 16 further takes therein the output of the iris encoder element 4 which detects the state of the iris 2. The output of the iris encoder element 4 is used as one of parameters necessary for optimum control over white balance adjustment and focus adjustment. More specifically, the control is based on the following: The depth of field is shallow if the position of the iris 2 is near to its full open position and is deep if it shows a small (stopped-down) aperture. Since the sensitivity of the focus control device for focus adjustment varies with the depth of field, it is necessary to vary its focus lens driving speed and the width of its dead zone according to the depth of field. Therefore, in order that the focus adjustment is stably, smoothly and quickly carried out, it is necessary to have information on the aperture value of the iris which is directly related to the depth of field.
In performing the white balance control, a change or no change in ambient shooting conditions is detected also through a change detected in the aperture value of the iris. According to the detected state of the iris, the control is performed, for example, to restart a white balance adjustment action or to predict the color temperature of a light source illuminating the object as a part of data for finding whether an image is being picked up outdoors or indoors. The aperture value thus gives an important parameter not only for the above-stated control but also for many other control actions of the camera.
However, the image sensor (a CCD in most cases) has recently come to function also as an electronic shutter to permit shooting control in a shutter priority AE mode in which the iris is controlled on the basis of an exposure time arbitrarily selected, or in a mode called "program mode" in which the iris, the AGC circuit and the electronic shutter function are automatically adjusted and controlled on the presumption of some of typical shooting conditions, such as a portrait, a landscape, a sport which shows a quick motion, etc. With these modes employed, the state (position) of the iris might vary with a difference in mode of shooting control even when the shooting object remains unchanged. In such a case, control over the white balance, focus, etc., which uses the information on the state of the iris as one of parameters in computing a correction value comes to be based on a misjudgment. The control thus becomes unstable, thereby degrading the performance of the camera. This problem has made it difficult, for an image pickup apparatus arranged to operate in these shooting control modes, to use an iris state detection signal in controlling the white balance and focus. This problem thus has been an impediment against an improvement in performance of each system related to the above-stated control.